M. Redolfi

Modelling of dust grain formation in a low-temperature plasma reactor used for simulating parasitic discharges expected under tokamak divertor domes

The presence of nanostructured dust particles has been reported in thermonuclear fusion reactors with carbon plasma-facing components. These particles contribute to tritium retention and pollution of the edge plasma. Understanding the way these particles can grow in the plasma phase is necessary for designing engineering solutions that avoid or at least limit their formation. As a first step towards this goal, this paper presents a numerical study of the formation of dust in a simple model laboratory electrical discharge: a dc discharge generated in argon with a graphite electrode. The aim here is to investigate whether carbon sputtering through ion and fast neutral bombardment of the cathode and subsequent molecular growth of carbon clusters and particle nucleation and development can explain dust formation in this model discharge. Results show that field reversal effects and negative cluster formation and trapping can fully explain dust formation in such a dc discharge.

Modeling carbonaceous particle formation in an argon graphite cathode dc discharge

We develop a model for the nucleation, growth and transport of carbonaceous dust particles in a non-reactive gas dc discharge where the carbon source is provided by cathode sputtering. We consider only the initial phase of the discharge when the dust charge density remains small with respect to the electron density. We find that an electric field reversal at the entrance of the negative glow region promotes trapping of negatively charged clusters and dust particles, confining them for long times in the plasma and favoring molecular growth. An essential ingredient for this process is electron attachment, which negatively charges the initially neutral clusters. We perform sensitivity studies on several number parameters: size of the largest molecular edifice, sticking coefficient, etc.

Experimental studies of the interactions between a hydrogen plasma and a carbon or tungsten wall

We present work done at LSPM (Laboratory of Sciences of Processes and Material Sciences), using the CASIMIR ECR plasma reactor device, aimed at answering questions about hydrogen isotope fuel retention and dust production in the context of the plasma-facing components (PFCs) of the International Thermonuclear Experimental Reactor (ITER). The plasma is characterized by means of optical spectroscopy, mass spectrometry and electrostatic probe; furthermore the dust density and size distribution will be measured by a laser diagnostic system. We present some early results obtained from hydrogen plasma exposure of pure tungsten samples, as well as samples of ITER-relevant tungsten-rich powders, produced inhouse by the ball-milling technique, which are likely to be a by-product of material erosion and migration during tokamak operation. In particular, we have performed measurements of the specific surface area of these powders as a proxy to their capacity to absorb hydrogen.

Spatial and temporal evolutions of ozone in a nanosecond pulse corona discharge at atmospheric pressure

This paper deals with the experimental determination of the spatial and temporal evolutions of the ozone concentration in an atmospheric pressure pulsed plasma, working in the nanosecond regime. We observed that ozone was produced in the localized region of the streamer. The ozone transport requires a characteristic time well above the millisecond. The numerical modelling of the streamer expansion confirms that the hydrodynamic expansion of the filamentary discharge region during the streamer propagation does not lead to a significant transport of atomic oxygen and ozone. It appears therefore that only diffusional transport can take place, which requires a characteristic time of the order of 50 ms.

A Simplified Global Model to Describe the Oxidation of Acetylene Under Nanosecond Pulsed Discharges in a Complex Corona Reactor

The objective of this paper is to analyse the oxidation of acetylene under nanosecond pulsed N2/O2 discharges generated in a complex multi-pin-to-plane (MPP) corona reactor in the frame of Yan’s generic chemical kinetic model. We made use of the results obtained from the detailed kinetic model published previously (Redolfi et al. in Plasma Chem Plasma Process 29(3):173–195, 2009) in order to propose a global reactor models based on Yan’s generic chemical model and taking into account the non-homogeneous and non-stationary character of the discharges. This enables us expressing the energy cost in terms of physical and kinetic parameters of the discharge. We checked the model validity by comparing predicted and measured energy cost-values for acetylene in MPP reactor. The methodology presented may be adapted to predict the energy cost in other complex corona reactor provided the model parameters are determined experimentally.

Particle Formation In A DC Discharge

We developed a model for the nucleation, growth and transport of carbonaceous dust particles in a non‐reactive gas dc discharge where the carbon source is provided by cathode sputtering. In a first part, we considered only the initial phase of the discharge when the dust charge density remains small with respect to the electron density. We found that an electric field reversal at the entrance of the negative glow region promotes trapping of negatively charged clusters and dust particles, confining them for long times in the plasma and favoring molecular growth. An essential ingredient for this process is electron attachment, which negatively charges the initially neutral clusters. We also showed that the field reversal mechanism can operate to trap negative clusters and particles under both electropositive and strongly electronegative plasma.